Ranking of Tornado Outbreaks across the United States and Their Climatological Characteristics CHRISTOPHER M. FUHRMANN,* CHARLES E. KONRAD II, AND MARGARET M. KOVACH Southeast Regional Climate Center, Department of Geography, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina JORDAN T. MCLEOD Climatology Research Laboratory, Department of Geography, The University of Georgia, Athens, Georgia WILLIAM G. SCHMITZ Southeast Regional Climate Center, Department of Geography, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina P. GRADY DIXON Department of Geosciences, Mississippi State University, Mississippi State, Mississippi (Manuscript received 30 October 2013, in final form 14 March 2014) ABSTRACT The calendar year 2011 was an extraordinary year for tornadoes across the United States, as it marked the second highest annual number of tornadoes since 1950 and was the deadliest tornado year since 1936. Most of the fatalities in 2011 occurred in a series of outbreaks, highlighted by a particularly strong outbreak across the southeastern United States in late April and a series of outbreaks over the Great Plains and Midwest regions in late May, which included a tornado rated as a category 5 event on the enhanced Fujita scale (EF5) that devastated the town of Joplin, Missouri. While most tornado-related fatalities often occur in outbreaks, very few studies have examined the climatological characteristics of outbreaks, particularly those of varying strength. In this study a straightforward metric to assess the strength, or physical magnitude, of tornado outbreaks east of the Rocky Mountains from 1973 to 2010 is developed. This measure of outbreak strength, which integrates the intensity of tornadoes [Fujita (F)/EF-scale rating] over their distance traveled (path- length), is more highly correlated with injuries and fatalities than other commonly used variables, such as the number of significant tornadoes, and is therefore more reflective of the potential threat of outbreaks to human life. All outbreaks are then ranked according to this metric and their climatological characteristics are ex- amined, with comparisons made to all other tornadoes not associated with outbreaks. The results of the ranking scheme are also compared to those of previous studies, while the strongest outbreaks from 2011 are ranked among other outbreaks in the modern record, including the April 1974 Super Outbreak. 1. Introduction The calendar year 2011 was an extraordinary year for tornadoes across the United States. Nearly 1700 torna- does were confirmed, making it the second highest annual total since 1950 (NOAA 2012). Most significantly, there were an estimated 553 tornado-related fatalities in 2011, resulting in the deadliest tornado year since 1936 and second deadliest since 1875 (SPC 2013). Most of * Current affiliation: Department of Geosciences, Mississippi State University, Mississippi State, Mississippi. Corresponding author address: Dr. Christopher M. Fuhrmann, Southeast Regional Climate Center, Dept. of Geography, Uni- versity of North Carolina at Chapel Hill, Saunders Hall, Campus Box 3220, Chapel Hill, NC 27599-3220. E-mail: [email protected]684 WEATHER AND FORECASTING VOLUME 29 DOI: 10.1175/WAF-D-13-00128.1 Ó 2014 American Meteorological Society
19
Embed
Ranking of Tornado Outbreaks across the United …...there were an estimated 553 tornado-related fatalities in 2011, resulting in the deadliest tornado year since 1936 and second deadliest
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Ranking of Tornado Outbreaks across the United States and Their ClimatologicalCharacteristics
CHRISTOPHER M. FUHRMANN,* CHARLES E. KONRAD II, AND MARGARET M. KOVACH
Southeast Regional Climate Center, Department of Geography, University of North Carolina at Chapel Hill,
Chapel Hill, North Carolina
JORDAN T. MCLEOD
Climatology Research Laboratory, Department of Geography, The University of Georgia, Athens, Georgia
WILLIAM G. SCHMITZ
Southeast Regional Climate Center, Department of Geography, University of North Carolina at Chapel Hill,
Chapel Hill, North Carolina
P. GRADY DIXON
Department of Geosciences, Mississippi State University, Mississippi State, Mississippi
(Manuscript received 30 October 2013, in final form 14 March 2014)
ABSTRACT
The calendar year 2011 was an extraordinary year for tornadoes across the United States, as it marked the
second highest annual number of tornadoes since 1950 and was the deadliest tornado year since 1936. Most of
the fatalities in 2011 occurred in a series of outbreaks, highlighted by a particularly strong outbreak across the
southeastern United States in late April and a series of outbreaks over the Great Plains and Midwest regions
in late May, which included a tornado rated as a category 5 event on the enhanced Fujita scale (EF5) that
devastated the town of Joplin, Missouri. While most tornado-related fatalities often occur in outbreaks, very
few studies have examined the climatological characteristics of outbreaks, particularly those of varying
strength. In this study a straightforward metric to assess the strength, or physical magnitude, of tornado
outbreaks east of the Rocky Mountains from 1973 to 2010 is developed. This measure of outbreak strength,
which integrates the intensity of tornadoes [Fujita (F)/EF-scale rating] over their distance traveled (path-
length), is more highly correlated with injuries and fatalities than other commonly used variables, such as the
number of significant tornadoes, and is thereforemore reflective of the potential threat of outbreaks to human
life. All outbreaks are then ranked according to this metric and their climatological characteristics are ex-
amined, with comparisons made to all other tornadoes not associated with outbreaks. The results of the
ranking scheme are also compared to those of previous studies, while the strongest outbreaks from 2011 are
ranked among other outbreaks in the modern record, including the April 1974 Super Outbreak.
1. Introduction
The calendar year 2011 was an extraordinary year for
tornadoes across the United States. Nearly 1700 torna-
does were confirmed, making it the second highest
annual total since 1950 (NOAA 2012). Most significantly,
there were an estimated 553 tornado-related fatalities in
2011, resulting in the deadliest tornado year since 1936
and second deadliest since 1875 (SPC 2013). Most of
*Current affiliation: Department of Geosciences, Mississippi
State University, Mississippi State, Mississippi.
Corresponding author address: Dr. Christopher M. Fuhrmann,
Southeast Regional Climate Center, Dept. of Geography, Uni-
versity of North Carolina at Chapel Hill, Saunders Hall, Campus
ated with outbreaks from 2006 to 2011 were randomly
selected from the database and categorized by maximum
F/EF-scale rating. To minimize potential geographical
reporting biases, tornadoes were sampled from 18 states
across the eastern United States. For each tornado, we
calculated the ratio of Fujita miles summed across each
segment of the tornado track to the number of Fujita
miles using the maximum F/EF-scale rating. This ratio
was then multiplied by the number of Fujita miles using
themaximumF/EF-scale rating to determine the number
of adjusted Fujita miles (AFMs) for each tornado. Fi-
nally, a mean adjustment factor for each F/EF-scale rat-
ing was computed by averaging all of the individual ratios
across the sample of tornadoes (Table 1). This adjustment
factor was then applied to all tornadoes in the study from
1973 to 2010, as well as those from 2011.
While path width has been used in other measures
of outbreak intensity [e.g., the Forbes impact index;
FIG. 2. Plots of the annual number of (a) tornado outbreaks and (b) Fujita miles (1mi 51.6 km) across the eastern two-thirds of the United States from 1954 to 2010. The solid line in
each figure represents a linear regression fit to the data and the two dotted lines represent the
95% confidence intervals for the regression coefficients.
aRankings (1–20) from Doswell et al. (2006) (D06) using the O index (period of record 1970–2003).bRankings (1–25) from Verbout et al. (2006) (V06) using the number of F1 and greater tornadoes (period of record 1954–2003).c Rankings using the Forbes outbreak index (FOI) (period of record is from pre-1950 to 2011) obtained from G. Forbes (2013, personal
communication).dRankings from Shafer and Doswell (2011) (SD11) using the N15 weighting (period of record 1960–2011).eRankings (1–25) from Shafer and Doswell (2012) (SD12) using the N15 weighting for multiday outbreaks (period of record 1960–2011).
Note that the rankings given for the 4–5May 2003 and 24–25May 2011 outbreaks are for the periods 29April–7May 2003 and 21–25May
2011, respectively.f Rankings using the DPI equation from Thompson and Vescio (1998).
696 WEATHER AND FORECAST ING VOLUME 29
the Hinton–Guthrie, Oklahoma, EF5 tornado (the first
F/EF5 tornado in Oklahoma in over 12 yr), resulting in
a top-10 ranking in the present study. The outbreak on
26–28 April will be discussed below. Two of the remain-
ing three outbreaks from 2011 (15 April and 16April; see
Fig. 1) ranked in the top 25, while the 22 May outbreak,
which included the Joplin, Missouri, EF5 tornado, ex-
hibited a top-75 ranking.
Approximately half of the outbreaks listed in Table 5
yielded consistent rankings among all of the studies.
Most of these outbreaks contained several moderate to
strong tornadoes with at least one long-track (i.e.,
greater than 100mi or 161 km), violent tornado (F/EF4
or F/EF5). The strongest outbreaks exhibited numer-
ous long-track, violent tornadoes (e.g., 3–4 April 1974,
31May 1985, 21–23 November 1992, and 4–5May 2003),
while other outbreaks exhibited just one or a few notable
tornadoes (e.g., 2–3 April 1982, Broken Bow, Oklahoma,
F5 tornado; 7–8 June 1984, Barneveld, Wisconsin, F5
tornado; 13–14 March 1990, Hesston–Goessel, Kansas,
F5 tornado; 26–27 April 1991, Andover, Kansas, F5 tor-
nado). The strongest outbreaks were also generally
widespread, including some that impacted several states
(e.g., 26–27 May 1973 outbreak, which covered 17 states
across the midwestern and southern United States). The
strength and lethality of the 3–4 April 1974 and 26–28
April 2011 outbreaks are particularly noteworthy, though
some differences do exist. The April 1974 outbreak ex-
hibited an unusually large number of long-track, violent
tornadoes over a relatively short amount of time [i.e., 30
tornadoes with an F4 or F5 rating over a 20-h period;
Locatelli et al. (2002)], while the April 2011 outbreak
exhibited more tornadoes of F/EF1 and greater strength
and had a total pathlength over 300km longer than the
April 1974 outbreak (Knupp et al. 2014).
While there was much general agreement among the
studies regarding the ranks of the strongest outbreaks,
there were also notable exceptions. In some cases, an
outbreak with a high number of AFMs exhibited very
few tornadoes overall or several weak tornadoes and
was therefore not ranked highly in some of the studies.
However, of those tornadoes that did form, most were
generally long track and violent. For example, only 13
tornadoes were reported in the 25–26 September 1973
outbreak, but six of those were rated F3 and greater,
with two tornadoes exhibiting pathlengths of over 100mi
(161 km) across parts of Oklahoma and Kansas. In
other cases, differences in the methods used to define
outbreaks had a significant bearing on the results. For
instance, studies that used the ‘‘tornado day’’ metric con-
sidered consecutive days of tornado activity separately,
while consecutive days of tornado activity would be
considered as one outbreak in the present study if there
was less than a 6-h period of tornado-free activity. An
example of this discrepancy was seen in the Red River
outbreak, which produced several strong and violent
tornadoes on both 10 and 11 April 1979, including the
Wichita Falls, Texas, F4 tornado on 10 April that killed
42 people. It is important to note that if an outbreak
was not listed in one of the previous studies, it was
deemed an unranked event even though it may have
been ranked just outside the listed outbreaks (e.g., the
outbreak was ranked 25 in the study though only the
top 20 outbreaks were listed).
Although a strong relationship was identified between
outbreak strength (i.e., adjusted Fujita miles) and the
number of fatalities (Table 2), there were some out-
breaks that resulted in a significant number of fatalities
despite exhibiting relatively few AFMs. Most of these
outbreaks were associated with a single violent, though
not necessarily long-track, tornado that occurred among
mostly weak tornadoes and tracked through highly pop-
ulated areas. Examples include the 27 May 1997 Jarrell,
Texas, F5 tornado (27 fatalities and 97 AFMs), and the 3
May 1999 Oklahoma City–Moore County, Oklahoma,
F5 tornado (40 fatalities and 889 AFMs). Some of these
events ranked highly in other studies (e.g., the 3–4 May
1999 event, which ranked 22nd in the present study, was
ranked 5th using the Forbes outbreak index, likely due
to the extraordinary financial costs). Ranking schemes
that consider only the number of reported tornadoes, or
emphasize other attributes such as path width and
damage amount, can produce results that differ from our
scheme, which emphasizes the strength of the tornadoes
integrated across the pathlength. For example, over 170
tornadoes were reported across the Great Plains and
Midwest from 29 to 31 May 2004, including an F4 tor-
nado with a maximum reported width of nearly 2.5mi
(4 km). As a result, this outbreak was ranked 13th when
using the Forbes outbreak index but ranked 63rd in
the present study due to the fact that the vast majority of
the tornadoes were weak and short lived. Similarly, a
total of 50 tornadoes of F1 and greater strength were
reported across the upper Midwest on 16 June 1992,
which ranked as the sixth largest big tornado day ac-
cording to Verbout et al. (2006). However, the longest
pathlength recorded among these tornadoes was 16mi
(26km) and therefore the outbreak received a ranking of
only 67th (577 AFMs) in the present study. Additionally,
because we only consider outbreaks since 1973, several
well-documented and deadly outbreaks were omitted
from the analysis, including the 1965 Palm Sunday
outbreak (over 270 fatalities) and the 1971 Mississippi
Delta outbreak (over 120 fatalities).
Tornado outbreaks may also be spawned by land-
falling tropical cyclones. Verbout et al. (2007) used the
JUNE 2014 FUHRMANN ET AL . 697
big-tornado-day metric to identify outbreaks associated
with tropical cyclones that made landfall along the U.S.
coast from 1954 to 2004. Most of these outbreaks con-
sisted of weak (F0 and F1) and short-lived tornadoes
resulting from tropical cyclones that made landfall along
the northern Gulf Coast. Of the 34 tornado-producing
tropical cyclones from 1973 to 2004 identified in Verbout
et al. (2007), only 4 spawned outbreaks with at least
100 AFMs. The strongest outbreak according to our
scheme was spawned by Hurricane Danny in August
1985 with 359 AFMs, followed by Hurricane Ivan in
September 2004 with 275 AFMs. Although both of
these tropical cyclones spawned over 100 tornadoes,
more than 80% were weak and short lived, therefore
resulting in relatively few AFMs.
5. Discussion and conclusions
In this study, we developed two related metrics (hec-
topascal miles and Fujita miles) to measure the physical
magnitude, or strength, of tornado outbreaks east of the
Rocky Mountains. To account for variations in tornado
strength along a given track, we calculated adjusted forms
of thesemetrics. The simplermetric, adjustedFujitamiles
(AFMs), was used to rank tornado outbreaks and ex-
amine their climatological and geographical characteris-
tics. Comparisons were also made between the results of
our ranking scheme and those of previous studies. Finally,
we assessed how the strongest outbreaks of 2011 ranked
among all outbreaks in the modern record (1973–2010).
Although there is no one best approach to defining
and ranking tornado outbreaks, our scheme accounts for
nonmeteorological trends in the data, is reproducible,
and yields rankings generally consistent with what others
would arrive at subjectively (Doswell et al. 2006). While
ranking schemes that incorporate multiple variables
may be considered more robust than schemes that use
a single combined variable (as in this study), our scheme
was intended to quantify the physical strength of out-
breaks and assess the relationship between the overall
risk (i.e., exposure) associated with outbreaks and the
resulting number of injuries and fatalities. This relation-
ship cannot be assessed using multivariate schemes that
incorporate injuries and fatalities as independent vari-
ables, such as those described inDoswell et al. (2006) and
Shafer and Doswell (2010, 2011). Moreover, ranking
schemes that include societal impacts are subject to the
vagaries of where tornadoes happen to occur (e.g., highly
populated versus sparsely populated areas). Neverthe-
less, we emphasize that this study is not attempting to
establish a universal definition or ranking scheme for
tornado outbreaks. Indeed, even minor changes in the
criteria used to define an outbreak, such as the time
between consecutive tornadoes in a sequence, will have
some bearing on the relative rankings.
Similar to previous studies (e.g., Galway 1977;
Schneider et al. 2004), we found that the vast majority
of tornado-related fatalities (approximately 80%) oc-
curred in outbreaks. In a given year, most tornado out-
breaks will be relatively weak and result in only a few
casualties. However, some outbreaks can be quite strong
and involve several violent, long-track tornadoes. For
instance, although outbreaks with at least 1000 AFMs
accounted for less than 3% of all outbreaks, they were
responsible for over one-third of all violent tornadoes
and over 30% of all tornado-related fatalities. Addi-
tionally, the range in strength among the strongest out-
breaks was quite considerable compared to the rest of
the outbreaks, which supports the findings of previous
studies that employed more sophisticated multivariate
ranking schemes to identify major tornado outbreaks
(Shafer and Doswell 2010, 2011). Using the Fujita miles
metric, we noted a strong positive relationship between
outbreak strength and the number of fatalities and in-
juries. This relationship was stronger than that using
other measures of outbreak strength, including the
destruction potential index and the number of F/EF2 and
greater tornadoes, suggesting that the Fujita miles metric
is more reflective of the potential threat of outbreaks to
human life. Therefore, a ranking scheme based on Fujita
miles may be useful to forecasters as guidance for cate-
gorizing the threat level for tornadoes [e.g., high, mod-
erate, and slight risk; see Shafer and Doswell (2010)]. An
evaluation of the utility of Fujita miles in an operational
setting is left to future work.
We noted climatological differences in the spatial and
temporal distributions of outbreak and nonoutbreak
tornadoes. The risk from outbreak tornadoes was great-
est across the lower Mississippi River valley, where sig-
nificant tornadoes are most common (see Figs. 6, 8, and
11 in Coleman and Dixon 2014). Coleman and Dixon
(2014) also found this area to be at greatest risk for sig-
nificant tornado pathlengths, although the area of maxi-
mum risk in their study extended farther east into
Alabama. The number of killer tornadoes is also
greatest in this region (Ashley 2007). Taken together,
the results of previous work and the present study
suggest that the Deep South is at greatest risk from
strong, long-track, killer tornadoes, and that these
tornadoes are most likely to occur in outbreaks. Non-
outbreak tornadoes also occurred frequently across the
lower Mississippi River valley, with a secondary maxi-
mum across part of the upper Midwest during the sum-
mer months. Outbreak tornadoes typically occurred
earlier in the spring season than nonoutbreak tornadoes
and were responsible for the secondary peak in tornado
698 WEATHER AND FORECAST ING VOLUME 29
frequency that has been documented in the fall season,
particularly across the Southeast (Brooks et al. 2003).
Because outbreak tornadoes are clustered in time (and
presumably in space), it is likely that they may be asso-
ciated with discrete synoptic-scale circulation patterns,
which could offer forecasters improved guidance and
predictability over nonoutbreak tornadoes. We intend to
examine this hypothesis in a follow-up study. Due to the
short 38-yr period of record used in the present study,
long-term trends in the frequency of outbreak and non-
outbreak tornadoes could not be assessed (Doswell
2007). Future work may consider using data compiled by
Grazulis (1993) for F2 and greater tornadoes extending
back to 1916 to identify relationships between outbreak
strength and climate variability. In addition, more re-
search is needed to understand the trends in outbreak
and nonoutbreak tornadoes, particularly over the past few
decades, as shown in this study. Such information could
help determine whether there has been a real change in
the behavior of tornadoes (e.g., an increasing number of
isolated tornadoes or smaller, weaker outbreaks) or
whether the trends are artificial (e.g., changes in reporting
practices and damage assessments).
We attempted to develop a ranking scheme that re-
lates the physical magnitude, or strength, of an outbreak
to the number of fatalities and injuries. Although there
can be large differences between the meteorological
significance of an outbreak and its societal impact, the
potential for destruction and loss of life (i.e., risk) is
greatest where exposure and intensity are maximized.
Our measure of outbreak strength (Fujita miles) em-
phasizes the cumulative risk associated with exposure to
tornadoes of varying intensity (i.e., pathlength and
F/EF-scale rating) integrated across all tornadoes in an
outbreak. The strong correlation between Fujita miles
and the number of fatalities and injuries suggests that
our measure of outbreak strength provides a reasonable
assessment of the potential lethality of outbreaks. How-
ever, we are not claiming that the Fujita miles metric is
the best way to rank the significance of tornado out-
breaks. Indeed, in some cases a particularly strong out-
break resulted in relatively few fatalities (e.g., the March
1990 Great Plains outbreak; Table 5), while outbreaks
consisting of only one or a few strong to violent tornadoes
may not have been ranked particularly high but still re-
sulted in significant damage and/or loss of life (e.g., the
outbreak associated with the Greensburg, Kansas, EF5
tornado in May 2007).
To assess how the use of different attributes affected
the ranking of tornado outbreaks, we compared the re-
sults of our ranking scheme for the top-20 outbreaks to
those of previous studies. We noted that the strongest
outbreaks generally yielded consistent rankings among
all of the studies. These outbreaks were of long duration
(i.e., at least 1 day); exhibited numerous tornadoes, in-
cluding several that were violent and long-track; and had
high numbers of casualties and significant property dam-
age. Therefore, the strongest and most destructive out-
breaks are likely to be ranked highly regardless of the
specific criteria. However, ranking schemes that consider
only the overall number of tornadoes (with no emphasis
placed on F/EF-scale rating) or use the tornado-day
metric are more likely to produce varying results. For
example, an outbreak consisting of numerous weak tor-
nadoes may be ranked higher than an outbreak with just
a few strong to violent tornadoes. In this case, a multi-
variate index that accounts for tornado frequency and
intensity can provide a more robust measure of out-
break strength (Doswell et al. 2006). In addition, using
the tornado-day metric, a period of generally continuous
tornado activity that extends beyond 24h will be con-
sidered as multiple outbreaks, and this may have some
bearing on the relative rankings (Shafer and Doswell
2012). We also note that there was much variability
between our rankings and those computed using the
destruction potential index. As noted in section 2, the
reporting of path width (which is used in the calculation
of the index) is not consistent in the historical tornado
database, although it is unclear what impact this had on
the rankings.
The present work was primarily motivated by the tor-
nado outbreaks of 2011, which contributed to the dead-
liest tornado year in theUnited States in 75 years.Most of
the tornado-related fatalities in 2011 occurred across the
southernUnited States from 26 to 28April in an outbreak
that has drawn comparisons to the April 1974 Super
Outbreak. Based on the Fujita miles metric for ranking
outbreaks by strength, we find these comparisons to be
valid, as both outbreaks ranked well above the next
strongest outbreaks in the historical record since 1973
(Fig. 3).While quantitativemeasures of outbreak strength
will likely rank one outbreak as stronger than the other
(e.g., Shafer and Doswell 2011, 2012; Doswell et al. 2012;
Knupp et al. 2014; Forbes impact index, G. Forbes 2013,
personal communication), those differences are insig-
nificant when considering the nature of tornado data
collection and, perhaps most importantly, the resulting
impacts to society and human life. Despite improve-
ments in the lead times of tornado warnings and the
ability to disseminate information to a wider segment
of the population in the 37 years between these two
outbreaks, the events of 2011 reminded us that major
tornado outbreaks, particularly those with violent and
long-track tornadoes, can still result in significant de-
struction and loss of life. Reducing our vulnerability
will require a better understanding of the relationships
JUNE 2014 FUHRMANN ET AL . 699
between the physical and societal aspects of tornado
outbreaks.
Acknowledgments.We thankGregCarbin of NOAA’s
Storm Prediction Center for his insights on the tornado
database and for his comments on an earlier version of
this study. We also thank Stuart Hinson of NOAA’s
National Climatic Data Center and Brent Macaloney of
NOAA’s National Weather Service for providing access
to the Performance Management and Data Verification
website. We greatly appreciate the efforts of Dr. Greg
Forbes, Dr. Chad Shafer, and one anonymous reviewer,
whose comments and suggestions significantly improved
themanuscript.We also thankDrs. Forbes and Shafer for
sharing their updated outbreak rankings. Funding for this
project was provided by NOAA as part of the Regional
Climate Center Program.
REFERENCES
Ashley, W. S., 2007: Spatial and temporal analysis of tornado fa-
talities in the United States: 1880–2005. Wea. Forecasting, 22,
1214–1228, doi:10.1175/2007WAF2007004.1.
Boruff, B. J., J. A. Easoz, S. D. Jones, H. R. Landry, J. D. Mitchem,
and S. L. Cutter, 2003: Tornado hazards in the United States.
Climate Res., 24, 103–117, doi:10.3354/cr024103.
Brooks, H. E., 2004: On the relationship of tornado path length and
width to intensity.Wea. Forecasting, 19, 310–319, doi:10.1175/
1520-0434(2004)019,0310:OTROTP.2.0.CO;2.
——, and C. A. Doswell III, 2001: Normalized damage from
major tornadoes in the United States: 1890–1999. Wea. Fore-
Copyright of Weather & Forecasting is the property of American Meteorological Society andits content may not be copied or emailed to multiple sites or posted to a listserv without thecopyright holder's express written permission. However, users may print, download, or emailarticles for individual use.